Titanium dioxide pigment particles with doped, dense SiO2 skin and methods for their manufacture

ABSTRACT

A method of predicting photostability of coatings with various dopants on titanium dioxide pigment particles is disclosed. Calculations of the density of states show that a doped coating which reduces the density of states near the band edge or increases the density of states within the band gap of the pigment particles increases the photostability of the doped pigment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority pursuant to 35 U.S.C. 119(e) of U.S.Provisional Application No. 60/772,919 filed Feb. 13, 2006, and toGerman applications DE102006004345.6 filed 30 Jan. 2006 andDE102006054988.8 filed 22 Nov. 2006 and is related to a secondapplication filed on the same date as the present application.

FIELD OF THE INVENTION

The invention relates to titanium dioxide pigment particles whosesurface is provided with a dense silicon dioxide skin doped with dopingelements, and methods for their manufacture. The titanium dioxidepigment particles according to the invention display improvedphotostability.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Because of its high refractive index, titanium dioxide is used as ahigh-quality pigment in many sectors, e.g. plastics, coatings, paper andfibres. However, titanium dioxide is photoactive, meaning that undesiredphotocatalytic reactions occur as a result of UV absorption, leading todegradation of the pigmented material [The Chemical Nature of Chalkingin the Presence of Titanium Dioxide Pigments, H. G. Völz, G. Kaempf, H.G. Fitzky, A. Klaeren, ACS Symp. Ser. 1981, 151, Photodegradation andPhotostabilization of Coatings].

In this context, titanium dioxide pigments absorb light in the nearultraviolet range, the result being that electron-hole pairs areproduced, which lead to the formation of highly reactive radicals on thetitanium dioxide surface. The radicals produced in this way result inbinder degradation in organic media. It is known from experimentalinvestigations that hydroxyl ions play a dominant role in thephotocatalytic process [Photocatalytic Degradation of Organic WaterContaminants: Mechanism Involving Hyroxyl Radical Attack, C. S. Turchi,D. F. Ollis, Journal of Catalysis 122, 1990, 178-192].

It is known that the photoactivity of TiO₂ can be reduced by doping theTiO₂ particles (e.g. with aluminium) or by means of inorganic surfacetreatment (e.g. by coating with oxides of silicon and/or aluminiumand/or zirconium) [Industrial Inorganic Pigments, ed. by G. Buxbaum,VCH, New York 1993, Seite 58-60]. In particular, several patentsdescribe the application of the most dense possible, amorphous layer ofSiO₂ to the particle surface, this being known as a “dense skin”. Thepurpose of this skin is to prevent the formation of free radicals on theparticle surface.

Wet-chemical methods for production of a dense SiO₂ skin, and of afurther Al₂O₃ coating on inorganic particles, particularly on TiO₂, aredescribed in patents U.S. Pat. No. 2,885,366, U.S. RE. 27,818 and U.S.Pat. No. 4,125,412. EP 0 245 984 B1 indicates a method which, as aresult of simultaneous addition of a solution containing Na₂SiO₃ and asolution containing B₂O₃, can be performed at relatively lowtemperatures of 65 to 90° C.

SiO₂ dense-skin treatments are also carried out in order to increase theabrasion resistance of glass fibres coated in this way and reduce theslipping properties of the fibres in the products manufactured. In thisconnection, U.S. Pat. No. 2,913,419 describes a wet-chemical method inwhich silicic acid is precipitated onto the particle surface togetherwith polyvalent metal ions, such as Cu, Ag, Ba, Mg, Be, Ca, Sr, Zn, Cd,Al, Ti, Zr, Sn, Pb, Cr, Mn, Co, Ni.

The method according to US 2006/0032402 makes it possible to increasethe photostability of dense-skin TiO₂ pigments. It is based on theincorporation of Sn, Ti or Zr in the SiO₂ skin applied by a wet-chemicalprocess.

In addition to the known wet-chemical methods for coating the surface ofTiO₂ particles, there are also dry-chemical methods in which the denseSiO₂ skin is deposited from the gas phase. In this case, during titaniumdioxide production by the chloride process, a silicon compound,preferably SiCl₄, is added to the TiO₂ particle stream with atemperature of over 1,000° C., such that a uniform, dense SiO₂ layer isformed on the particle surface.

EP 1 042 408 B1 describes a gas-phase method for surface coating with Siand B, P, Mg, Nb or Ge oxide.

SUMMARY OF THE INVENTION

The object is solved by titanium dioxide pigment particles whose surfaceis coated with a dense SiO₂ skin deposited from the gas phase and dopedwith at least one doping element, whereby the doping element is selectedfrom the groupconsisting of Sn, Sb, In, Y, Zn, F, Mn, Cu, Mo, Cd, Ce, Wand Bi as well as mixtures thereof.

The object is furthermore solved by titanium dioxide pigment particleswhose surface is coated with a dense SiO₂ skin produced in awet-chemical process and doped with at least one doping element, wherebythe doping element is selected from the group consisting of Sb, In, Ge,Y, Nb, F, Mo, Ce, W and Bi as well as mixtures thereof.

The object is furthermore solved by a method for manufacturing titaniumdioxide pigment particles whose surface is coated with a dense SiO₂ skindoped with at least one doping element, comprising the steps:

a) Reaction, in the gas phase, of titanium tetrachloride with analuminium halide and a gas containing oxygen in a reactor at atemperature over 1,000° C., in order to create a particle streamcontaining TiO₂ particles,

b) Contacting of the particle stream with at least two compounds, wherethe first compound is a silicon oxide precursor compound and the secondcompound is selected from the group consisting of oxide precursorcompounds of Sn, Sb, In, Y, Zn, Mn, Cu, Mo, Cd, Ce, W, Bi and precursorcompounds of F as well as mixtures thereof,

c) Cooling of the particle stream, in order to create pigment particlesthat are coated with a dense SiO₂ skin doped with at least one dopingelement, wherein the doping elements are selected from thegroupconsisting of Sn, Sb, In, Y, Zn, F, Mn, Cu, Mo, Cd, Ce, W and Bi aswell as mixtures thereof.

Finally, a further solution to the object consists in a method formanufacturing titanium dioxide pigment particles whose surface is coatedwith a dense SiO₂ skin doped with at least one doping element,comprising the steps:

a) Provision of an aqueous suspension of TiO₂ particles with a pH valuein excess of 10,

b) Addition of an aqueous solution of an alkaline silicon component andat least one aqueous solution of a component containing a dopingelement, wherein the doping element is selected from the groupconsistingof Sb, In, Ge, Y, Nb, F, Mo, Ce W and Bi as well as mixtures thereof.

c) Deposition of a dense SiO₂ skin doped with at least one dopingelement on the surface of the particles by lowering the pH value of thesuspension to a value below 9, preferably to below 8, where the dopingelements are selected from the groupconsisting of Sb, In, Ge, Y, Nb, F,Mo, Ce, W and Bi as well as mixtures thereof.

Further advantageous embodiments of the invention are indicated in thesub-claims.

The subject matter of the invention is coated titanium dioxide pigmentsthat are further improved in terms of their photostability.

DETAILED DESCRIPTION OF THE INVENTION

The pigments according to the invention contain, in a dense skin on thetitanium dioxide particle surface, preferably 0.1 to 6.0% by weight, andmore preferably 0.2 to 4.0% by weight, silicon, calculated as SiO₂, andpreferably 0.01 to 3.0% by weight, and more preferably 0.05 to 2.0% byweight, doping elements, calculated as oxide or, in the case of F, aselement and referred to the total pigment.

In a preferred embodiment, the particles are coated with an additionallayer of 0.5 to 6.0% by weight, more preferably 1.0 to 4.0% by weight,aluminium oxide or hydrous aluminium oxide, calculated as Al₂O₃ andreferred to the total pigment.

The titanium dioxide particles are preferably rutile.

Here and below, “doping element” is to be taken to mean the respectiveelement as atom or ion or a respective compound like an oxide, whereappropriate. In the context of the description of the coatings producedby the wet-chemical process, the term “oxide” is to be taken, here andbelow, to also mean the corresponding hydrous oxides or correspondinghydrates. All data disclosed below regarding pH value, temperature,concentration in % by weight or % by volume, etc., are to be interpretedas including all values lying in the range of the respective measuringaccuracy known to the person skilled in the art.

The invention is based on the fact that, in order to increase thephotostability, the photocatalytic process must be interrupted in asuitable manner, i.e. that the production of highly reactive radicals byexcited electron-hole pairs must be made more difficult. This can beachieved by utilising various mechanisms, e.g. by increasing therecombination rate of the electron-hole pairs, or by building up anenergetic barrier on the pigment surface.

A dense and uniformly applied SiO₂ skin already builds up an energeticbarrier on the TiO₂ surface, as detectable by a reduced energy statedensity near the band edge in the valence band and in the conductionband of the coated TiO₂ surface, compared to the untreated TiO₂ surface.Surprisingly, doping of the SiO₂ skin with selected elements leads to afurther reduction in the energy state densities near the band edge, thusraising the energetic barrier and thus further improving thephotostability of the TiO₂ pigment coated in this way.

Additional energy states within the band gap between the valence bandand conduction band promote the recombination of electron-hole pairs.Doping of the SiO₂ layer with selected elements generates these energystates and thus effects also an improvement in photostability comparedto an undoped SiO₂ layer.

The elements Sn, Sb, In, Ge, Y, Zr, Zn, Nb, F, Mn, Cu, Mo, Cd, Ce, W,and Bi have proven to be suitable doping components. The doped SiO₂ skincan be applied both by the wet-chemical method and by the gas-phasemethod. It is, however, known that the gas-phase method is generallycapable of applying a more uniform skin than the wet-chemical method.Other elements not yet calculated are also anticipated by the inventors,and can be found by ordinary experimentation with computer calculationas shown in this specification. All such elements as have not yet beenfound by physical and chemical experimentation are claimed in thisinvention. The effective doping elements found so far, which areexcluded in the claims, are doping elements selected from the groupconsisting of Ag, Al, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Mg, Mn, Ni, Pb, Sn,Sr, Ti, Zn, and Zr for the wet chemical process, and the doping elementsselected from the group consisting of Al, B, Ge, Mg, Nb, P, Zr for thedry process.

In addition, combinations of elements used as dopants can also be foundby computer calculation of energy state densities and band gap states.The combination of two or more dopants can interact and produce a resultwhich is more than either of the two dopants separately, and suchsynergetic combinations in the composition ranges necessary are foundeasily by methods of the present invention, and would be very difficultand time consuming to find by ordinary experimentation by methodsintroduced in the prior art.

An example of the invention is described below with the help of FIGS. 1to 18.

FIG. 1 shows the energy states at the transition from the atom to thesolid (taken from: P. A. Cox, “The Electronic Structure and Chemistry ofSolids”, Oxford Science Publications 1987, p. 13).

FIG. 2 shows the energy state density of the TiO₂ surface without andwith SiO₂ coating.

FIG. 3 shows the energy state density of the TiO₂ surface with SiO₂coating and with Sn-doped SiO₂ coating.

FIG. 4 shows the energy state density of the TiO₂ surface with SiO₂coating and with Sb-doped SiO₂ coating.

FIG. 5 shows the energy state density of the TiO₂ surface with SiO₂coating and with In-doped SiO₂ coating.

FIG. 6 shows the energy state density of the TiO₂ surface with SiO₂coating and with Ge-doped SiO₂ coating.

FIG. 7 shows the energy state density of the TiO₂ surface with SiO₂coating and with Y-doped SiO₂ coating.

FIG. 8 shows the energy state density of the TiO₂ surface with SiO₂coating and with Nb-doped SiO₂ coating.

FIG. 9 shows the energy state density of the TiO₂ surface with SiO₂coating and with F-doped SiO₂ coating.

FIG. 10 shows the energy state density of the TiO₂ surface with SiO₂coating and with Mn-doped SiO₂ coating.

FIG. 11 shows the energy state density of the TiO₂ surface with SiO₂coating and with Cu-doped SiO₂ coating.

FIG. 12 shows the energy state density of the TiO₂ surface with SiO₂coating and with Mo-doped SiO₂ coating.

FIG. 13 shows the energy state density of the TiO₂ surface with SiO₂coating and with Cd-doped SiO coating.

FIG. 14 shows the energy state density of the TiO₂ surface with coatingand with Ce-doped SiO₂ coating.

FIG. 15 shows the energy state density of the TiO₂ surface with SiO₂coating and with W-doped SiO₂ coating.

FIG. 16 shows the energy state density of the TiO₂ surface with SiO₂coating and with Bi-doped SiO₂ coating.

FIG. 17 shows the energy state density of the TiO₂ surface with SiO₂coating and with Mg-doped SiO₂ coating.

FIG. 18 shows the energy state density of the TiO₂ surface with SiO₂coating and with Al-doped SiO₂ coating.

The energy state densities were calculated quantum-mechanically with thehelp of the CASTEP software package (Version 4.6, 1 Jun. 2001) fromAccelrys Inc., San Diego. The calculations were performed using theCASTEP density functional code in the LDA (local density approximation).Detailed information has been published by V. Milman et al. in:International Journal of Quant. Chemistry 77 (2000), p. 895 to 910.

The following valence states, including the semi-core states, were usedfor titanium: 3s, 3p, 3d, 4s and 4p. The valence states 2s and 2p wereused for oxygen, and the valence states 3s and 3p for silicon. For thedoping elements, the semi-core states 4d or 4s and 4p or 2p wereincluded for indium, yttrium and magnesium. The basic set used for thedoping elements was as follows:

Sn: 5s, 5p, 6s, 6p, 7s

Sb: 5s, 5p, 6s, 6p, 7s

In: 4d, 5s, 5p, 6s, 6p, 7s

Ge: 4s, 4p, 4d

Y: 4s, 4p, 4d, 5s, 5p

Nb: 4s, 4p, 4d, 5s, 5p

F: 2s, 2p

Mn: 3d, 4s, 4p

Cu: 3d, 4s, 4p

Mo: 4s, 4p, 4d, 5s, 5p

Cd: 4d, 5s, 5p, 6s, 6p

Ce: 4f, 5s, 5p, 6s, 6p, 7s, 7p, 8s

W: 5d, 6s, 6p

Bi: 6s, 6p, 7s, 7p, 8s

Mg: 2p, 3s, 3p

Al: 3s, 3p

The kinetic energy cut-off for the plane waves was 380 eV. Structuralgeometric optimisation was not performed, since the mathematical modelcould be evaluated and confirmed on the basis of known experimentalresults (coating with Sn, Al, Zr and Zn). Thus, the model calculationsyield sufficient accuracy for examination of the photostability.

The state density calculations were based on a grid according to theMonkhorst-Pack scheme. The surface calculations were performed inaccordance with the “slab model method” with a vacuum thickness of 10 Å.

EXAMPLES

The invention is explained on the basis of Examples 1 to 14 (doping ofthe SiO₂ layer with one of the doping elements Sn, Sb, In, Ge, Y, Nb, F,Mn, Cu, Mo, Cd, Ce, W, Bi), Comparative Example 1 (pure SiO₂ layer),Comparative Example 2 (doping of the SiO₂ layer with Mg) and ComparativeExample 3 (doping of the SiO₂ layer with Al).

The calculation for Comparative Example 1 is based on complete coverageof a TiO₂ (110) surface with an SiO₂ monolayer. In this context, theunit cell comprises 52 atoms (Ti₈Si₈O₃₆). Applied to the pigment, thecalculated monomolecular coverage with SiO₂ with a layer thickness ofapproximately 0.2 nm corresponds to a percentage by weight of roughly0.3% by weight SiO₂, referred to TiO₂.

The percentage by weight was calculated on the basis of the followingvalues: typical value of the specific surface (to BET) for TiO₂particles manufactured by the chloride process: 6.2 m²/g; thickness ofthe monomolecular layer: 0.2 nm; density of the SiO₂ layer: 2.2 g/cm³.

Examples 1 to 14 and Comparative Examples 2 and 3 describe coverage ofthe TiO₂ surface with a monomolecular SiO₂ layer doped at an atomicratio of 1 (doping element X):7 (Si), i.e. the unit cell comprisesTi₈Si₇X₁O₃₆. Applied to the TiO₂ pigment, this results in the followingpercentages by weight of the doping elements, calculated as oxide andreferred to TiO₂:

Example 1: roughly 0.10% by weight SnO₂,

Example 2: roughly 0.09% by weight Sb₂O₃,

Example 3: roughly 0.09% by weight In₂O₃,

Example 4: roughly 0.07% by weight GeO₂,

Example 5: roughly 0.14% by weight Y₂O₃,

Example 6: roughly 0.09% by weight Nb₂O₅,

Example 7: roughly 0.01% by weight F,

Example 8: roughly 0.06% by weight MnO₂,

Example 9: roughly 0.06% by weight CuO,

Example 10: roughly 0.10% by weight MoO₃,

Example 11: roughly 0.09% by weight CdO,

Example 12: roughly 0.12% by weight CeO₂,

Example 13: roughly 0.16% by weight WO₃,

Example 14: roughly 0.09% by weight Bi₂O₃,

Reference Example 2: roughly 0.03% by weight MgO,

Reference Example 3: roughly 0.04% by weight Al₂O₃.

Results

The result of the quantum-mechanical CASTEP calculations is theelectronic structure. This can be analysed in the form of bandstructures (energy bands spatially resolved) or the state densities(integrated energy states).

FIG. 1 shows a simplified block diagram (d) of the electronic structure.The block diagram reflects only the energy bandwidth and position of theenergy band. The state density (e) is used for the energy statedistribution within the energy band.

FIG. 2 shows the effect of a pure, undoped SiO₂ coating (ComparativeExample 1) on the photoactivity of the TiO₂: the calculated statedensity of the pure TiO₂ (110) surface is shown as a broken line, thatof the SiO₂-coated surface as a solid line. The positive effect of theSiO₂ coating on photostability is partly based on the reduction of thestate density in the conduction band (CB) near the band edge, comparedto the uncoated TiO₂ surface, this reducing the transfer ofelectron-hole pairs to the surrounding matrix. At the same time, thepositive effect is intensified by the fact that there is additionally areduction in the state density near the band edge in the valence band(VB).

FIG. 3 shows the effect of doping the SiO₂ layer with Sn (Example 1) onthe state densities, compared to the pure SiO₂ coating. In this case,there is a further reduction in the VB state density near the band edge,this leading to improved photostability.

FIGS. 4 to 8 show the respective effect of doping the SiO₂ layer with Sb(Example 2, FIG. 4), In (Example 3, FIG. 5), Ge (Example 4, FIG. 6), Y(Example 5, FIG. 7) and Nb (Example 6, FIG. 8). Surprisingly, areduction in the VB state density near the band edge can be seen in eachcase, meaning that these coatings lead to an increase in photostability.Similar doping of the SiO₂ layer with the elements Zr or Zn likewiseleads to improved stability compared to an undoped SiO₂ layer.

FIGS. 9 to 16 show the respective effect of doping the SiO₂ layer with F(Example 7, FIG. 9), Mn (Example 8, FIG. 10), Cu (Example 9, FIG. 11),Mo (Example 10, FIG. 12), Cd (Example 11, FIG. 13), Ce (Example 12, FIG.14), W (Example 13, FIG. 15) and Bi (Example 14, FIG. 16). Doping of theSiO₂ layer with F, Mn, Cu, Mo, Cd, Ce, W or Bi surprisingly leads toadditional energy states within the band gap which serve asrecombination centers for electron hole pairs and thus to an improvedphotostability.

FIG. 17 shows the effect of doping the SiO₂ layer with Mg (ComparativeExample 2) on the energy state densities. In this case, there is anincrease in the VB state density near the band edge, meaning that dopingof the SiO₂ layer with Mg results in a loss of photostability.

FIG. 18 shows the effect of doping the SiO₂ layer with Al (ComparativeExample 3) on the energy state densities. In this case again, there isan increase in the VB state density near the band edge, meaning thatdoping of the SiO₂ layer with Al results in a loss of photostability.

The results of the state density calculations correlate precisely withthe measurements of photostability in the experimentally doped samples.Thus, the calculations can be used to predict the usefulness of theelemental dopants without the much more difficult and time consumingtrial and error experiments of trying to incorporate the dopants in thedense skins, and then measuring the photostability. One of skill in theart may use the results of the present specification to calculate andpredict the results of any other dopant elements not mentionedspecifically in this specification, for which the calculations have notyet been completed. Use of such dopant elements is claimed in thisapplication, excluding only those dopant elements which have beenexperimentally found and published. The dopant elements known to theinventors which have previously been experimentally found and publishedare, for the dry process, Ag, Al, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Mg, Mn,Ni, Pb, Sn, Sr, Ti, Zn, and Zr, and for the wet process, Al, B, Ge, Mg,Nb, P, and Zr. The inventors state that the results of the wet processmay not necessarily be used to predict the results of the dry processes,and vice versa, so that separate applications are necessary for eachprocess.

Process Control

Methods for coating titanium dioxide particles with dense SiO₂ as suchare known. The traditional processes work via the aqueous phase. To thisend, a TiO₂ particle suspension is produced, mixed with a dispersantwhere appropriate, and wet-milled where appropriate. The dense SiO₂ skinis customarily precipitated by adding alkali metal-silicate solutionsand appropriate pH value control.

The doping element is added in the form of a salt solution, togetherwith the silicate solution or separately before or after addition of thesilicate solution. The person skilled in the art is familiar with thesuitable compounds and necessary quantities for controlling the pH valuein order to produce a dense skin.

Doping of the dense SiO₂ skin according to the invention can, forexample, be achieved by adding the following salts to the suspension,where this compilation is not to be interpreted as a restriction of theinvention.

Doping with Sb: antimony chloride, antimony chloride oxide, antimonyfluoride, antimony sulphate

Doping with In: indium chloride, indium sulphate

Doping with Ge: germanium chloride, germanates

Doping with Y: yttrium chloride, yttrium fluoride

Doping with Nb: niobium chloride, niobates

Doping with F: flourine hydrogen, fluorides

Doping with Mn: manganese chloride, manganese sulphate

Doping with Cu: copper chloride, copper sulphate

Doping with Mo: molybdenum chloride, molybdates

Doping with Cd: cadmium chloride, cadmium sulphate

Doping with Ce: cerium nitrate, cerium sulphate

Doping with W: wolframates

Doping with Bi: bismuth nitrate, bismuth sulphate

In a particularly preferred embodiment, an outer layer of hydrousaluminium oxide is additionally applied to the particles by knownmethods.

In another embodiment of the invention, the dense SiO₂ skin is depositedon the particle surface from the gas phase. Various methods are knownfor this purpose. For example, coating can be performed in a fluidisedbed at temperatures below roughly 1,000° C. Methods of this kind aredescribed in U.S. Pat. No. 3,552,995, GB 1 330 157 or US 2001 0041217A1. Alternatively, coating takes place in a tubular reactor directlyfollowing formation of the TiO₂ particles in the chloride process; thesemethods are described, for example, in patents or patent applications WO98/036441 A1, EP 0 767 759 B1, EP 1042 408 B1 and WO 01/081410 A2. Forcoating in a tubular reactor, the precursor compound used for the SiO₂is customarily a silicon halide, particularly SiCl₄, which is generallyintroduced downstream of the point where the reactants TiCl₄ and AlCl₃are combined with the oxygen-containing gas. For instance, WO 01/081410A2 indicates that the silicon halide is added at a point where the TiO₂formation reaction is at least 97% complete. In any case, thetemperatures at the point of introduction should be above 1,000° C.,preferably above 1,200° C. The SiO₂ precursor compound is oxidised anddeposited on the surface of the TiO₂ particles in the form of a densesilicon dioxide skin. In contrast to the wet-chemical method, water andhydrate-free oxide layers are formed during gas-phase treatment, theseadsorbing hydroxyl ions and water molecules only on the surface.

The doping element is likewise added to the particle stream as aprecursor compound, either in parallel with the SiO₂ precursor compound,or upstream or downstream. Here, too, the temperature of the particlestream at the point of introduction must be above 1,000° C., preferablyabove 1,200° C. The following compounds are suitable precursor compoundsfor the various doping metal oxides, although this compilation is not tobe interpreted as a restriction of the invention:

Doping with Sn: tin halide, such as tin chloride

Doping with Sb: antimony halide, such as antimony chloride

Doping with In: indium halide, such as indium chloride

Doping with Y: yttrium halide, such as yttrium chloride

Doping with Zr: zirconium halide, such as zirconium chloride

Doping with Zn: zinc halide, such as zinc chloride

Doping with Nb: niobium halide, such as niobium chloride

Doping with F: fluorine, fluorine hydrogen, fluorides

Doping with Mn: manganese chloride

Doping with Cu: copper chloride

Doping with Mo: molybdenum chloride

Doping with Cd: cadmium chloride

Doping with Ce: cerium chloride

Doping with W: tungsten chloride

Doping with Bi: bismuth chloride

In a particularly preferred embodiment, an outer layer of aluminiumoxide is additionally applied to the particles by introducing a suitablealuminium oxide precursor compound, such as AlCl₃, into the particlestream farther downstream.

Finally, the titanium dioxide pigments provided with the doped, denseSiO₂ skin can be further treated by known methods, regardless of whetherthey were coated in a suspension or in the gas phase. For example,further inorganic layers of one or more metal oxides can be applied.Moreover, further surface treatment with nitrate and/or organic surfacetreatment can be performed. The compounds known to the person skilled inthe art for organic surface treatment of titanium dioxide pigmentparticles are also suitable for organic surface treatment of theparticles according to the invention, e.g. organosilanes,organosiloxanes, organophosphonates, etc., or polyalcohols, such astrimethylethane (TME) or trimethylpropane (TMP), etc.

The titanium dioxide pigment particles according to the invention aresuitable for use in plastics, paints, coatings and papers. They can alsobe used as a starting basis for a suspension for producing paper orcoatings, for example.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. All of theabove identified U.S. provisional applications, patents, and referencematerial, including references contained therein, are herebyincorporated herein by reference in their entirety.

1. Titanium dioxide (TiO₂) pigment particles, comprising: a) TiO₂ coreparticles; b) a dense silicon dioxide (SiO₂) skin covering the coreparticles, the dense skin produced in a dry process, the dense skindoped with at least one doping element, wherein the at least one dopingelement is selected from the group consisting of Sn, Sb, In, Y, Zn, F,Mn, Cu, Mo, Cd, Ce, W and Bi, as well as mixtures thereof.
 2. The TiO₂pigment particles of claim 1, further comprising; c) a further coatingof aluminium oxide or hydrous aluminium oxide upon the dense skincovering.
 3. The TiO₂ pigment particles of claim 2, wherein thealuminium content of the further coating is 0.5 to 6.0% by weightcalculated as Al₂O₃ and referred to the total pigment.
 4. The TiO₂pigment particles of claim 3, wherein the aluminium content of thefurther coating is 1.0 to 4.0% by weight.
 5. The TiO₂ pigment particlesof claim 1, wherein the silicon content of the dense skin is 0.1 to 6.0%by weight calculated as SiO₂ and referred to the total pigment.
 6. TheTiO₂ pigment particles of claim 5, wherein the the silicon content ofthe dense skin is 0.2 to 4.0% by weight, calculated as SiO₂ and referredto the total pigment.
 7. The TiO₂ pigment particles of claim 1, whereinthe content of doping elements in the dense skin is 0.01 to 3.0% byweight calculated as oxide and in the case of F calculated as element.8. The TiO₂ pigment particles of claim 7, wherein the content of dopingelements in the dense skin is 0.05 to 2.0% by weight.
 9. A method formanufacturing TiO₂ pigment particles whose surface is coated with adense SiO₂ skin doped with at least one doping element, comprising thesteps: a) reacting titanium tetrachloride with an aluminium halide and agas containing oxygen in a gas phase reactor at a temperature above1,000° C., thereby creating a gas stream containing TiO₂ particles; b)contacting the particle containing gas stream with at least twocompounds, where a first compound is a silicon oxide precursor compoundand a second compound is selected from the group consisting of oxideprecursor compounds of Sn, Sb, In, Y, Zn, Mn, Cu, Mo, Cd, Ce, W, Bi andprecursor compounds of F as well as mixtures thereof, c) cooling theparticle stream, thereby creating pigment particles that are coated witha dense SiO₂ skin deposited on the TiO₂ core particles, the dense SiO₂skin doped with at least one doping element.
 10. The method of claim 9,further comprising; d) adding a further layer of aluminium oxide orhydrous aluminium oxide on the dense SiO₂ skin, the further layer addedby either a dry process or a wet chemical process.
 11. The method ofclaim 10, further comprising; e) adding further layer of organicmaterial on the aluminium oxide or hydrous aluminium oxide layer in awet-chemical process.
 12. The method of claim 9, wherein the siliconcontent of the dense skin is 0.1 to 6.0% by weight calculated as SiO₂and referred to the total pigment.
 13. The method of claim 12, whereinthe the silicon content of the dense skin is 0.2 to 4.0% by weight,calculated as SiO₂ and referred to the total pigment.
 14. The method ofclaim 9, wherein the content of doping elements in the dense skin is0.01 to 3.0% by weight calculated as oxide and in the case of Fcalculated as element.
 15. The method of claim 14, wherein the contentof doping elements in the dense skin is 0.05 to 2.0% by weightcalculated as oxide and in the case of F calculated as element.
 16. Themethod of claim 9, further comprising; d) adding the TiO₂ pigmentparticles produced in step c) to a process for making plastics, paints,coatings or papers.
 17. The method of claim 10, further comprising; e)adding the TiO₂ pigment particles produced in step d) to a process formaking plastics, paints, coatings or papers.
 18. The method of claim 11,further comprising; f) adding the TiO₂ pigment particles produced instep e) to a process for making plastics, paints, coatings or paper. 19.Titanium dioxide (TiO₂) pigment particles, comprising: a) TiO₂ coreparticles; b) a dense silicon dioxide (SiO₂) skin covering the core, thedense skin produced in a dry process, the dense skin doped with at leastone doping element, wherein the at least one doping element eitherreduces the density of states near the band edge or creates additionalstates in the band gap of the material of the dense skin, excludingdoping elements selected from the group consisting of Al, B, Ge, Mg, Nb,P, and Zr.